Investigators concluded David A. Burke, a former USAir ticket agent, smuggled a revolver aboard the flight from Los Angeles to San Francisco. (Associated Press)

An FBI investigation into the mysterious crash of a Pacific Southwest Airlines jetliner near Paso Robles focused Tuesday on a recently dismissed airline employee who was believed to have boarded the flight with a handgun, intent on killing the man who fired him.

Authorities confirmed that both a Los Angeles International Airport-based supervisor for USAir and an employee who was fired by the airline last month were passengers aboard Flight 1771, and a source close to the investigation described the bizarre revenge murder scenario as "a very, very good theory."

The former employee was identified as David A. Burke, 35, who had worked for USAir for 15 years before his dismissal last month, allegedly after he was caught by a hidden camera stealing receipts from in-flight cocktail sales. Burke served in a variety of terminal-based jobs, and at the time of his firing was a ticket agent.

The manager was identified as Ray F. Thomson, 48, who headed USAir's office in Terminal One at LAX. Thomson lived in Tiburon, a bay-side city just north of San Francisco, and commuted to his Southern California job.

Police Investigators

FBI agents and police investigators twice visited Burke's Long Beach condominium Tuesday. The first time, they were accompanied by a woman described as Burke's girlfriend. The woman was believed to have given authorities their initial information that Burke, incensed over his Nov. 18 firing, had boarded Flight 1771 with a .44-caliber Magnum pistol and could be culpable in the crash.

At 8:30 p.m. investigators returned to the residence with a search warrant. FBI agents at the home declined to talk to reporters.

Forty-three people were aboard the PSA BAe 146-200, a British-built, four-engine jetliner, late Monday afternoon when it plummeted nose-first from 22,000 feet into a rugged hillside near Paso Robles in San Luis Obispo County. No one survived. The plane had been bound from Los Angeles to San Francisco.

Report of Gunfire

Foul play was considered an investigative possibility from the outset. The pilot broadcast a report of gunfire in the passenger compartment moments before the crash. Additionally, a National Transportation Safety Board official said Tuesday night that a secret cockpit distress signal had been activated, apparently by pilot Gregg N. Lindamood, and flashed to air traffic controllers "a minute or two" before impact.

Richard T. Bretzing, special agent in charge of the FBI office in Los Angeles, said that there was "a substantially increased basis for concluding that, in fact, criminal activity did bring this aircraft down. And that," Bretzing told a press conference held one-half mile from the crash site, "is based not only on the tape that the air traffic controllers have, but on other investigations we are conducting outside of this area."

He would not elaborate.

Investigators refused comment on a report by ABC-TV, quoting an unnamed government source, that Burke had left behind a suicide message. Bretzing said only that search warrants might be served, if necessary.

One of Burke's co-workers, who asked not to be identified, said Burke had been seen at the USAir offices earlier Monday, but apparently left after talking with Thomson's subordinates. Thomson was not present at the time, but the early visit could resolve the question of how Burke knew which flight his former supervisor was taking.

Proving the revenge murder scenario with forensic evidence could be exceedingly difficult, given what authorities described as the "utter destruction" of the plane and its passengers on impact. Moreover, the theory, if accurate, would leave investigators and aviation officials with other troublesome questions: How could shots fired from within bring down an airliner? And how could a former employee, said by USAir officials to have been stripped of his work badge, slip past airport security with a handgun?

Thousands of Pieces

A team of 100 searchers started at dawn Tuesday the monumental task of locating and identifying wreckage and remains. They combed the steep incline where the airliner, after what was described by witnesses as a screaming, 45-degree dive, slammed to earth and exploded into thousands of tiny pieces.

"We're looking for a weapon," Bretzing had said early in the morning, "and hopefully if there's one we'll find it."

No weapon, however, had been found by 12:30 p.m., when the search was suspended because a rainstorm turned the crash site into a quagmire, raising fears that valuable evidence might be trampled.

Pieces of aircraft and human remains that littered the crash site were tagged with fluorescent orange tape and recorded on a grid chart. The body parts were to be removed when the weather cleared and taken to a temporary morgue. Witnesses who ran to the crash said there were no human remains larger than a hand.

"I was hoping there would be somebody alive," said Paul Wiley, a water well driller who lives near the crash sight and saw the jetliner's dive, "but once you got over there, there was no chance of that."

He said the diving aircraft "sounded like it was breaking the sound barrier" as it screamed toward earth, but appeared to be intact during the plunge.

A Different Message

Wiley's observation, could prove meaningful, officials said. "A plane coming down intact tells us a different message than a plane coming down disintegrating in its fall," Bretzing said. "It tells us that possibly there was no explosion."

Patricia A. Goldman, vice chairman of the NTSB, told reporters Tuesday night that the largest piece of wreckage she saw at the sight was "only a couple of feet long." Nonetheless, Goldman said, the pattern of the wreckage could yield telling evidence as to what happened in the last minutes of the flight.

"The pattern will help determine the plane's attitude when it struck," she said, "whether there was breakup before it struck, whether there was fire or explosion on board. . . . The impact angle might give an indication of how the plane went down, whether there was pilot input."

One scenario that investigators were to pursue was whether the pilot and first officer might have been struck by bullets from behind, driving them forward into the aircraft's control sticks. This could explain the unusually steep and steady angle at which witnesses said the jetliner dived toward the ground.

The aircraft's cockpit voice recorder has been recovered, and NTSB officials were reviewing it Tuesday night in Washington. The recorder tapes cockpit conversations and conceivably could document whether there was gunfire in the cockpit, and if the crew members were struck.

Aviation experts said it seemed unlikely that damage to the plane from bullets fired in the passenger compartment could cause the aircraft to crash because of the redundancy built into the plane's control systems.

Richard H. Wood, a private aircraft accident investigator who directs the University of Southern California's Aviation Safety Program, suggested a number of possibilities. "Perhaps a bullet went through the fuselage and into a wing fuel tank and ignited a fire," said Wood, who stressed he was offering purely hypothetical scenarios. "Or, you can pick an obvious one, in which the plane goes down if you shoot two critical components--the pilots."

PSA officials refused to reveal Tuesday where Burke or Thomson were sitting in the aircraft, saying that the information had been turned over to investigators.

Authorities familiar with security procedures at LAX said it is not uncommon for airline personnel to be waved around metal detector checkpoints used to electronically search passengers for weapons.

"In some cases, employees do show a badge and go around security," said Margery Craig, a PSA spokeswoman.

Officials at USAir, which merged with PSA in a $400-million deal in May, said that Burke's plastic identification card was recovered when he was dismissed Nov. 18 and that the card was destroyed.

A USAir official said the carrier's policy is to allow uniformed employees with identification badges to skirt around airport metal detectors.

Burke worked for USAir as a uniformed customer service agent, and co-workers said he was a familiar face around Terminal One--suggesting this might have assisted him in slipping by security even without a proper badge.

"It's always possible to get through if they know your face," said one USAir worker, who asked not to be identified.

The co-worker also suggested that Burke could know how to operate the combination locks on doors that limit access through the terminal to the Tarmac, and conceivably could have boarded the airplane through that direction.

Burke's former colleagues and his current neighbors uniformly expressed surprise that he might be implicated in the air crash. His co-workers said he had a reputation for being, as one put it, "very intelligent and very cool." And people who lived near his modest, three-bedroom condominium on West Spring Street in Long Beach described the stocky son of a Rochester, N.Y., cabdriver as "always very nice, congenial."

Allegedly Caught Stealing

According to co-workers and company officials, Burke transferred to Los Angeles in late 1986 after working 14 years for the airline in Rochester, N.Y.

Officials in the Los Angeles city attorney's office said allegations that Burke stole $69 from a USAir office in Los Angeles on Nov. 15 led to consideration of misdemeanor charges against him.

"Our people were supposed to meet with police today (Tuesday) to discuss the case," Mike Qualls, a spokesman for the city attorney's office, said Tuesday.

Thomson was described by employees as a firm but fair supervisor. His wife, Dorothy, works as a flight attendant for American Airlines.

Private security guards kept watch on their split-level house Tuesday. Police there would say only that Thomson had never reported being threatened by Burke.

"He had been on his way up," neighbor Frank Buchanan said of Thomson. "What a tragic thing."

Names of Victims

Throughout the night Monday and into Tuesday morning PSA was releasing the names of passengers known to be aboard Flight 1771 as soon as their relatives were notified. Among the five crew members and 38 passengers lost in the crash were James Sylla, 53, president of Chevron USA Inc., and three other executives of the oil company. They were Owen F. Murphy, 60, a Los Angeles regional vice president, Jocelyn G. Kempe, 56, public affairs manager for Ventura and Santa Barbara, and Allen F. Swanson, 45, public affairs manager in Orange County.

Douglas Arthur, PSA's chief pilot, also was riding as a passenger on the flight. Arthur, 41, joined PSA in 1975 and had logged 7,500 flight hours, a spokesman said.

Airline officials waited until Tuesday to release Arthur's name because his wife, PSA director of reservations Nikki St. Germain, could not be located. St. Germain's brother, Donald St. Germain, was a PSA flight attendant who died in a 1978 San Diego air crash that claimed 144 lives.

Wolfgang Studeman, a prominent West German scientist, was also aboard. Studeman, a scientist with the Max Planck Institute for Aeronomy in Lindau, West Germany, was traveling to a meeting of the American Geophysical Union in San Francisco.

Peter H. King reported from Los Angeles and Eric Malnic from San Luis Obispo. Also contributing to this story were Times staff writers David Freed and Robert W. Stewart in Paso Robles; H.G. Reza in San Diego; Dan Morain in Tiburon; Mark A. Stein in San Francisco; Robert E. Dallos and Eileen Quigley in New York; Robert L. Jackson and Ronald J. Ostrow in Washington, and Maura Dolan, Paul Feldman, Daryl Kelley, Terry Pristin and James Rainey in Los Angeles.